Marine seismic exploration is the study of the subsurface of the earth underneath bodies of water. A marine seismic acquisition system is used to acquire marine seismic data. The seismic acquisition system includes a source, which initiates seismic waves, sensors, which detect seismic waves, and other components. The seismic waves propagate from the source through the water and into the subsurface of the earth where they are reflected and refracted. Some reflected waves travel back up through the water and are detected by the sensors of the acquisition system, converted into electrical signals, and recorded as seismic data. The data are subsequently processed and analyzed to estimate attributes of the earth's subsurface such as the shape and position of geological structures, properties of the rocks and pore fluids, and others. This information is often used for making hydrocarbon and mineral exploration decisions.
FIGS. 1 and 2 show example systems involving marine seismic surveying. In FIG. 1, a ship 801 tows a seismic source 802 several meters below the surface 803 of the ocean. The seismic source 802 is activated to produce a down-going seismic wave 804d that is at least partially reflected by a subsea interface or boundary 805 below the surface of the seafloor. The up-going earth reflected seismic wave 804u then travels toward a platform, cable, or streamer 807 that has one or more sensors 806. Although not shown, the streamer 807 may include an array of streamers having sensors. The sensors 806 may include pressure sensors, pressure gradient sensors, or motion sensors; which may include hydrophones, and may also include geophones. The sensors 806 may be separate stations having internal memory or may be connected to a recording system typically on a vessel for receiving output of the measuring devices transmitted to the vessel.
FIG. 2 shows an alternative example of marine seismic surveying. A first ship 901 tows a seismic source 902 several meters below the surface 903 of the ocean. The seismic source 902 is activated to produce a down-going seismic wave 904d that is at least partially reflected by a subsea interface or boundary 905 below the surface of the seafloor. The up-going reflected seismic wave 904u then travels toward a platform, cable array, or cable 907 attached to a second ship 908 and having one or more sensors 906. The cable 907 may be one or more ocean bottom cables that are arranged stationary on the seafloor 909. Similar to the sensors 806, these sensors 906 may include pressure sensors, pressure gradient sensors, or motion sensors; which may include hydrophones, and may also include geophones. The sensors 906 may be separate stations having internal memory or may be connected to a recording system typically on vessel 908 for receiving output of the measuring devices transmitted to the vessel. When necessary, the second ship 908 is used to move the cable 907 to a new position on the seafloor 909. Several miles of cable 907 are typically deployed along the seafloor 909, and several cables are typically deployed in parallel arrangements. Cable 907 arrangements are particularly well suited for use in certain zones (such as zones cluttered with platforms or where the water is very shallow) and where the use of ship-towed hydrophone arrays (which are located proximate the ocean surface 903 and are typically referred to as “streamers”) are not practical.
Typically, the sources and receivers of a marine seismic acquisition system are located beneath and near the sea surface. The sea surface is a boundary between water and air and is highly reflective to seismic waves. Reflections from the sea surface cause interference, a phenomenon often referred to as “ghosting”. For example, when a source is activated, waves propagate out from the source in many directions. Down-going waves propagate through the water towards the earth's subsurface. However, there are also up-going waves that reflect off the sea surface then propagate downward through the water, combining with the waves down-going directly from the source. This phenomenon is often referred to as “source ghosting”. Source ghosting modulates the source's amplitude spectrum reducing the amount of information available in the seismic data, particularly at and near the “notch frequencies” of the source ghosting function. (The Fourier Transform of a time function, a(t), gives the “frequency spectrum”, A(f), which may be written as A(f)=|A(f)|ejφ(f), where |A(f)| is called the “amplitude spectrum” and φ(f) is called the “phase spectrum”.)
FIG. 3 shows the amplitude spectrum of a ghost function for a monopole source located at a depth d below the sea surface. The ghost function is given by 1−exp(−j4πfd/c), which may equivalently be written 2j sin(2πfd/c)exp(−j2πfd/c), where c is the seismic wave propagation velocity in the water. Ghosting modulates the amplitude spectrum of the source, |A(f)|, by 2|sin(2πfd/c)|. The notch frequencies are those frequencies at the local minima in the amplitude spectrum of the ghost function, the local minima given by fn=nc/2d, n=0, 1, 2, . . . . FIG. 4 shows the amplitude spectrum of a ghost function for a dipole source located at a depth d below the sea surface. The ghost function is given by 1+exp(−j4πfd/c), which may equivalently be written 2 cos(2πfd/c)exp(−j2πfd/c). Ghosting modulates the amplitude spectrum of the source, |A(f)|, by 2|cos(2πfd/c)|. The notch frequencies are those frequencies at the local minima in the amplitude spectrum of the ghost function, the local minima given by fn=(1+2n)c/4d, n=0, 1, 2, . . . .
A similar phenomenon exists on the sensor side of the acquisition system. In this case, sensors are measuring the seismic waves propagating upward from the earth's subsurface towards the sensor. The wave propagates by the sensor, continues to propagate to the sea surface, then reflects back to the sensor. So the wave measured by the sensor is a combination of the up-going wave and its down-going sea surface reflection, a phenomenon often referred to as “sensor ghosting”. Sensor ghosting modulates the amplitude spectrum of the wave from the subsurface, further reducing the amount of information available in the seismic data, particularly at and near the notch frequencies of the sensor ghosting function. FIGS. 5 and 6 show examples of sensor ghost function amplitude spectra and notch frequencies.
FIG. 5 shows the amplitude spectrum of a ghost function for a pressure sensor located at a depth d below the sea surface. The ghost function is given by 1−exp(−j4πfd/c), which may equivalently be written 2j sin(2πfd/c)exp(−j2πfd/c). Ghosting modulates the amplitude spectrum of the earth's reflections by 2|sin(2πfd/c)|. The notch frequencies are those frequencies at the local minima in the amplitude spectrum of the ghost function, the local minima given by fn=nc/2d, n=0, 1, 2 . . . . FIG. 6 shows the amplitude spectrum of a ghost function for a pressure gradient or motion sensor located at a depth d below the sea surface. The ghost function is given by 1+exp(−j4πfd/c), which may equivalently be written 2 cos(2πfd/c)exp(−j2πfd/c). Ghosting modulates the amplitude spectrum of the earth's reflections by 2|cos(2πfd/c)|. The notch frequencies are those frequencies at the local minima in the amplitude spectrum of the ghost function, the local minima given by fn=(1+2n)c/4d, n=0, 1, 2 . . . .
An acquisition method that is useful to removing sensor ghosting has been commercially available for many years. The method employs two sensor types and is commonly referred to as two-component seismic acquisition, or 2C seismic acquisition. Herein, the method is referred to as “two-component sensor seismic acquisition,” or “2C-sensor seismic acquisition,” to distinguish it from the present invention. The two sensor types are preferably a sensor for detecting pressure and a sensor for detecting pressure gradient. Because of the difficulty of measuring pressure gradient directly, a measurement of motion, such as displacement, velocity, or acceleration is often used as proxy. Measurement of both pressure and pressure gradient (or motion as a proxy) provides sufficient information to determine a separation between the up-going and down-going wavefields, a process known as wavefield separation. [Ref: Ramirez and Weglein, “Green's theorem as a comprehensive framework for data reconstruction, regularization, wavefield separation, seismic interferometry, and wavelet estimation: A tutorial,” Geophysics 74, no. 6, W35-W62 (2009).] Typically, pressure sensors are hydrophones, and motion sensors are geophones that measure particle velocity or accelerometers that measure acceleration. The ghosting function for a pressure sensor is different than that for a pressure gradient or motion sensor, the notch frequencies of one occurring at the peak frequencies of the other. Use of both sensor types to measure the wavefield makes it possible to retain information in the seismic data that otherwise would be lost due to sensor ghosting effects if a single sensor type were used. Seismic data from the two sensor types can be combined to remove the sensor ghosting effect, or “sensor de-ghost” the seismic data.
The present invention provides a technology that gives benefits similar to 2C-sensor seismic acquisition, but for the source side. The invention uses two source types to acquire seismic data without source ghosting, or to acquire seismic data containing sufficient information to enable the effective removal of source ghosting.